Apparatus and method for testing the insertion loss of electrical filters

ABSTRACT

An apparatus for testing electrical filters is disclosed which includes an enclosed cavity made up of a plurality of sections joined to one another with plug-in connections. The cavity has a feed-in capacitor section at each end for introducing a low frequency current representative of a rated load current for a filter under test. The feed-in capacitor sections are each joined to one end of an inductor section, one of which inductor sections represents a power source impedance and the other inductor section representing a load impedance. The inductor sections are each connected at their opposite ends to a test section, one of which introduces high frequency signals representing electromagnetic interference into the cavity, and the other being used for measuring voltage across the load impedance. The cavity is completed by a filter section disposed between the two test sections which houses a filter under test. The complete circuit also includes a source of low frequency current to represent rated filter load, a high frequency generator attached to the test section that introduces high frequencies to the cavity, means for measuring high frequency current delivered to the circuit, and a detector connected to the other test section for reading measured voltage of the load impedance. Use of the apparatus involves a method of testing in which high frequency signals are introduced in parallel with the filter under test and the power source impedance, and measurements of the high frequency source current are made with the filter both in and removed from the circuit for a fixed load impedance voltage, from which current readings the filter insertion loss is calculated.

United States Patent McMartin et al.

[451 Mar. 21, 1972 [54] APPARATUS AND METHOD FOR TESTING THE INSERTIONLOSS OF ELECTRICAL FILTERS [72] Inventors: William J. McMartin, CedarRapids, Iowa;

I-lans E. Weidmann, Glendale, Wis.

[52] U.S. Cl. .324/57 R, 324/58 R [51] lnt.Cl ..G01r27/00,G0lr 27/04[58] Field ofSearch ..324/57,58,'57 A,58 A, 78 J,

[56] References Cited UNITED STATES PATENTS 3,025,462 3/1962 King..324/58 3,419,799 12/1968 Papadeas.... 2,746,015 5/1956 Alsberg..324/58 A OTHER PUBLICATIONS Schlicke H. M. and Weidmann H. CompatibleEMI Filters in E E SPEFIELQQQJMMQIKLJ1 Primary Examiner-Alfred E. SmithAttorney-Arthur H. Seidel and Thomas O, Kloehn [5 7] ABSTRACT Anapparatus for testing electrical filters is disclosed which includes anenclosed cavity made up of a plurality of sections joined to one anotherwith plug-in connections. The cavity has a feed-in capacitor section ateach end for introducing a low frequency current representative of arated load current for a filter under test. The feed-in capacitorsections are each joined to one end of an inductor section, one of whichinductor sections represents a power source impedance and the otherinductor section representing a load impedance. The inductor sectionsare each connected at their opposite ends to a test section, one ofwhich introduces high frequency signals representing electromagneticinterference into the cavity, and the other being used for measuringvoltage across the load impedance. The cavity is completed by a filtersection disposed between the two test sections which houses a filterunder test. The complete circuit also includes a source of low frequencycurrent to represent rated filter load, a high frequency generatorattached to the test section that introduces high frequencies to thecavity, means for measuring high frequency current delivered to thecircuit, and a detector connected to the other test section for readingmeasured voltage of the load impedance. Use of the apparatus involves amethod of testing in which high frequency signals are introduced inparallel with the filter under test and the power source impedance, andmeasurements of the high frequency source current are made with thefilter both in and removed from the circuit for a fixed load impedancevoltage, from which current readings the filter insertion loss iscalculated.

22 Claims, 2 Drawing Figures //4 EMI FILTER V V i i T A A T i v A E A 9i GENERATOR I 7 FILTER :DETECTOR E M I l CURRENT Z2 Z3 E z/ Z0 I Z 2 Z iLI 1 i l l l I l i PATENTEDHIIRZI I972 SHEEI 1 OF 2 INVENTORS WILLIAM J.MC MARTIN HANS E.WEIDMANN Ml M .PZMEEDU mOIFUukwQ luk zu ATTORNEYPATENTEDMARZ] I972 3.651.400

SHEET 2 OF 2 w INVENTORS WILLIAM J. MC MARTIN HANS E. WEIDMANN BZZMVM/ATTORNEY APPARATUS AND METHOD FOR TESTING TI-IE INSERTION LOSS OFELECTRICAL FILTERS BACKGROUND OF THE INVENTION There has been a need forsatisfactory testing of electrical filters that are inserted between anelectrical apparatus and a power source for the purpose of attenuatinghigh frequency interference signals appearing on the power supply lines.Most test methods for determining the insertion loss of such power linefilters have been deficient, for the reason they prescribe fixed sourceand load impedance values for connection to the filter, and such fixedvalues are not representative of actual operating conditions when thepower source is a commercial power supply, or one that feeds severalloads.

Typical test procedures deal with filters for use between preselected,fixed power supply impedances and fixed load impedances. A filterdesigned to provide an adequate insertion loss over the frequency rangeof electromagnetic interference between preselected, fixed power sourceand load impedances may not function adequately, and may even bedetrimental, when used for eliminating electromagnetic interferenceappearing on commercial power supply lines. This is for the reason thatimpedance values presented by commercial power supplies vary with theswitching of other loads off and on the line and with frequency. Theinteraction of these variable impedance values at the interface, orconnection with a filter is determinative of the insertion loss providedby the filter. In some cases of impedance mismatch benveen a filter andthe power source and load impedances to which it is connected the filtermay lose all attenuating capability, and may even present a negativeinsertion loss.

It is necessary that a more satisfactory test be provided for filtersused to eliminate electromagnetic interference from electrical apparatusconnected to unregulated power lines presenting varying impedance valuesat the junctures with the filters. Such test should indicate throughouta frequency range what the insertion loss of the filter will be underconditions of worst mismatch between the filter and the power source andload impedances. The worst-case condition, for a given frequency, occurswhen the reactive component of the power source, or the load is equaland opposite in value to that of the filter. A resonant condition thenarises at the interface between filter and source, or load, and theeffectiveness of the filter may sharply decrease. Since impedances ofmany power supplies are not predictable, and even vary, the filterperformance under the worst-case condition for each frequency in therange of intended use should be determinable by test. Filter design canthen be perfected to meet prescribed minimum insertion loss values overa preselected range of frequencies, and it is an object of the presentinvention'to provide a satisfactory test apparatus for this purpose.

SUMMARY OF THE INVENTION The present invention provides a circuit havingan inductor, representing a power source impedance, and a high frequencycurrent source in parallel with one another, these two elements of animpedance and current source being connectable to the input of a filterunder test. The power source impedance is readily substituted byimpedances of other values to enable tests to be made over a range offrequencies, and the circuit further contemplates use of a secondimpedance to simulate a load impedance and a low frequency currentsource to provide rated load currents for the filter, which current mayhave a biasing effect upon filter operation.

A preferred form of the invention houses the source and load impedances,the filter under test, capacitors for feeding in low frequency current,and test connections for introducing and measuring high frequencies inindividual casings that are readily connectable with one another byplug-in technique. When assembled together, they provide an enclosedcavity in which the filter and source and load impedances are adequatelyshielded to thereby obtain accurate measurements. The individual casingsprovide for quick and ready substitution of different impedance valuesinto the circuit and for ease of removal of a filter, so that the testprocedure can be easily carried out. To further achieve the objective orreliable operation, test sections are provided in which direct metal tometal connections are made between test leads and the conductors joiningthe filter with the source and load impedances. Magnetically coupledprobes for injecting high frequency and making measurements areconsequently eliminated.

For obtaining worst-case conditions for a pi-type filter withcapacitance in the shunt legs it is desirable to strive for pureinductors as representing source and load impedances. Resistance dampensresonance, and since a resonant condition at the interface between afilter and its power source and load connections represents theworst-case condition, the test apparatus should have a high Q for theinductors at frequencies in the vicinity of the worst-case condition. AQ of approximately 15 is satisfactory for this purpose, and it is anobjective to provide a test circuit having a high Q interfacialconnection between a filter and test circuit elements.

A further objective is to provide a high frequency current generator,that simulates electromagnetic interference, that approximates an idealcurrent source. Such a source has a high impedance that does not impairinterfacial resonance between the filter under test and the high Qinductor simulating a power source impedance when connected directly tothe filter and inductor. This simplifies the manner of introducing thehigh frequency and makes the circuit easier to construct and to use.Another test circuit of our design has been disclosed in IEEE SPECTRUM,Vol. 4, No. 10, Oct. 1967, pp. 59-68. There, injection and detectionprobes are magnetically coupled into the circuit by transformers, andthe high frequency source is a voltage source serially connected withthe simulated power source impedance. Such circuit is difficult toconstruct and use. Also, the present invention further departs byemploying easily connected plug-in sections that provide an enclosedcavity when assembled. The frequency range over which an apparatus ofthe present invention is usable is also broader than for the priorcircuit.

A further objective is to conduct a low frequency, or DC load currentthrough the filter and test impedances to simulate actual operatingconditions of the filter. Fifty amperes is presently typical of suchload current flow, and the connections between the sections making upthe test cavity must be sufficient to handle such values. Other objectsare to provide a circuit that has a simple test procedure, that isrugged to withstand repeated handling of the circuit components, andwhich gives reliable results. The method of the test procedure ischaracterized by inserting fixed high Q inductors into the circuit, thenvarying the frequency of the electromagnetic interference signals untilworst-case condition is met by observation of load impedance voltage,and then measuring the high frequency input current. Next, the filter isremoved and measurement is made of the high frequency current for thesame voltage across the load impedance. In this method, the high Qinductors are not complicated by being variable, and frequency variationis obtained indirectly by setting the current generator at a worst-casecondition for each inductor value.

The foregoing and other objects and advantages of the invention willappear from the following description, in which description there isshown by way of illustration and not of limitation a preferredembodiment. Such embodiment does not represent the full scope of theinvention, but rather the invention may be employed in many differentembodiments, and reference is made to the claims for interpreting thescope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings is a schematicwiring diagram of a circuit embodying the invention, and

FIG. 2 is a view of individual sections that make up an enclosed cavityforming a part of the invention, in which some of such sections areshown in cross section and in which the sections are disconnected fromone another to make more clear their individual constructions,

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1 of thedrawings, a cavity 1 is represented by a solid line that encloses anumber of circuit components, and this cavity 1 is shown as beingsub-divided into a number of sections by clefts 2 in the solid linerepresentation of the cavity. From left to right in FIG. 1, thesesections comprise a capacitor section 3 for introducing a low frequencyload current into the cavity 1, an inductor section 4 which simulates apower source impedance, a test section 5 for the introduction of highfrequencies simulating electromagnetic interference, a filter section 6that houses a filter under test, a second test section 7 for measuringvoltage across a load impedance, a second inductor section 8 thatsimulates a load impedance, and a second capacitor section 9. Thephysical construction of these sections 3-9 will be describedhereinafter in connection with F IG. 2.

Extending from the left-hand capacitor section 3 is a lead 10 thatconnects through a resistance 11 to a variable transformer 12. Thetransformer 12 is connected to a load current source 13 which provides alow frequency source to simulate the power delivered from a power sourcewhich is to be passed through a filter under test. The term lowfrequency source, as used herein also encompasses a direct currentsource, and it is distinguished from electromagnetic interferencefrequencies that are to be attenuated by a filter. The load currentsource 13 is isolated from the cavity 1 by locating the cavity 1 in ashielded room 14 that is represented by dotted lines. The load currentfed to the cavity through the lead 10 passes from the right-handcapacitor section 9 through a variable load resistance 15 By adjustmentof the load resistance 15 and the transformer 12 the low frequencycurrent is set to a rated value for a filter under test. Fifty amperesis representative of such a value for present power line filters, andthe various sections 3 9 must be capable of transmitting such current.The resistor 11 is provided to eliminate any possible resonance betweenthe transformer 12 and the capacitance within the capacitor section 3.

To introduce a high frequency representative of electromagneticinterference into the cavity 1 a generator 16 is connected to the testsection 5. The output of the generator 16 is fed to a power amplifier 17and then through a high pass filter 18 to the test section 5. Thepurpose of the high pass filter 18 is to block low frequency current ofthe load current source 13 from entering the amplifier l7 and generator16. Co-axial connections are provided between the elements 16, 17 and 18and the test section 5, and for the purpose of making currentmeasurements a standard current probe 19 is coupled with the coaxialline leading to the test section 5.

The second test section 7 has a lead 20 passing outward from the cavity1 through a resistor 21 and a high pass filter 22 for connection with adetector 23. Such detector 23 measures the voltage within the cavity 1that appears across an inductor within the second inductor section 8.The purpose of the resistor 21 is to preserve a high 0 within the cavity1, and the high pass filter 22 serves to block low frequency currentfrom entering the detector 23.

Referring now to FIG, 2, and commencing with the lefthand capacitorsection 3, there is shown a feed'in capacitor 24 that has a rectangular,block-shaped metallic exterior 25 which provides a shielded enclosure. Afeed-through line 26 threaded at its opposite ends enters the shieldedexterior 25 at the left-hand side and re-emerges at the righthand side.The capacitor electrodes (not shown) of the capacitor 24 areelectrically positioned between the feed-through line 26 and theshielding exterior 25, so that high frequencies representingelectromagnetic interference which appear on the feedthrough line 26will be shunted to the exterior 25, and low frequencies introduced atone end of the feed-through line 26 will pass directly to the other endof the line 26.

0n the right-hand face of the capacitor 24 there is bolted a maleconnector ring 27 that is concentric with and encircles the right-handend of the feed-through line 26. This connector ring 27 presents a malerim 28 and a sealing face 29 adjacent the rim 28. Located inside theconnector ring 27 is a cup shaped connection hub 30 tat constitutes aninner male connector member. This hub 30 is in threaded connection withthe right-hand end of the feed-through line 26, and it is spaced fromthe shielding exterior 25 by an insulating ring 31.

Adjacent the capacitor section 3 is the inductor section 4. The section4 has a metallic, circular cylindrical outer casing 32 that shields theinterior and which constitutes part of the enclosing wall of thecavity 1. A female connector ring 33 is tightly fitted within theleft-hand end of the casing 32, and it presents a sealing face 34 thatfaces axially toward and is of the same diameter as the sealing face 29of the capacitor section 3. The radially inner edge of the sealing face34 forms a small ledge 35, and seated behind the ledge 35 is a ring ofclosely spaced, resilient, metallic fingers 36. The resilient fingers 36are formed from a metal strip, they are interconnected to one another,and such strip is bent into a circle for insertion behind the ledge 35.The fingers 36 together with the connector ring 33 form an outer femalesocket for tight receipt of the rim 28 of the male connector ring 27.

Mounted within the ring 33 is a flat, circular insulator 37 thatsupports a female inner hub 38. The inner hub 38 is held concentric withrespect to the outer casing 32, and to hold the parts 37 and 38 inposition a number of split retaining rings 39 are employed. The hub 38is cup shaped, and has a lip 40 behind which is seated a ring ofresilient fingers 41, such fingers 41 and the lip 40 form an innerfemale socket adapted to receive and tightly grasp the male hub 30 ofthe capacitor section 3.

At the right-hand end of the outer casing 32 a male connector ring 42presenting a sealing face 43 which is immediately adjacent an outer rim44. A supporting insulator 45 is mounted in the ring 42 to support amale inner hub 46 that is concentric with the outer casing 32. The hub46 is generally cup shaped and presents an inner rim 47 that lies in acommon transverse plane with the outer rim 44. To complete the inductorsection 4, there is a coil 48 connected to and spanning between the hubs38 and 46. This coil 48 presents a high Q inductive reactance andsimulates a power source impedance.

Turning now to the test section 5, there is an outer casing 49 ofrelatively short axial length. In the casing 49 there are snugly fittedfemale connector ring 50 and male connector ring 51, which in crosssection are of like geometry as the rings 33 and 42 of the inductorsection 4. Supporting insulators 52 and 53 mount a female inner hub 54and a male inner hub 55. The hub 54 presents an inner female socket likethe inner socket of the section 4, and the hub 55 presents a maleconnection like the hub 46 of section 4. The hubs 54, 55 are of an axialdepth such that they abut one another, and the interfaces are solderedto one another to develop a unitary central member that presentsnegligible resistance and inductance to the circuit.

A test wire 59 is soldered at its lower end to the hub 55 in a directmetal to metal contact, and extends radially outward through the casin g49 to a standard coaxial connector 60. The exterior of the connector 60joins electrically with the casing 49 through the medium of a short,upstanding tube 61.

The filter section 6 constitutes a chamber defined by an outer casing62. A filter 63, that is under test, is mounted within the chamber by acup 64 that is tightly fitted within the left-hand end of the casing 62.The filter 63 is inserted through and bolted to the central web 65 ofthe cup 64, and the web 65 constitutes an electrical connection betweenthe filter 63 and the casing 62, so that one side of the filter isgrounded in the same manner as in actual use. The cup 64 also functionsto provide a sealing face 66 and to seat a ring of resilient fingers 67to present an outer female socket similarly as the sockets for the priordescribed sections 4,

The filter 63 has a feed-through lead which presents threaded terminals68 at its opposite ends, and tightly secured to the left-hand terminal68 is a female inner hub 69. Mounted on the opposite end of the filter63 is a male inner hub 70, and by virtue of the rigid connection of thefilter 63 to the web 65 the hubs 69, 70 are firmly held in position. Tocomplete the section 6, a male connector ring 71 presenting a rim 72 isseated in the right-hand end of the casing 62. The ring 71 is brazed tothe casing 62, and similarly the prior described rings 33, 42, S0, S1and cup 64 are permanently fixed in place by brazing.

The second test section 7 is like test section 5, and its female socketsare connectable with the male portions at the righthand end of thefilter section 6. Next, there is shown in FIG. 2 the second inductorsection 8, and for purposes of illustration this section 8 differs fromthe first inductor section 4. The overall length is reduced, and theinductor element 73 constitutes a short rod held in place by a pair ofbolts. This illustrates that to achieve different values of inductivereactance the inductor elements take different configurations. A shortrod, as in section 8, presents a small reactance, while inductors ofmultiple turns present increased reactance.

A further distinction between the inductor sections 4 and 8, is that thesection 8 has female connectors at each end. This is a matter of designchoice, and if preferred the last section 9 may have female connectors,in which case the inductor section 8 will have male connectors at itsright-hand end like those of section 4. As illustrated in FIG. 2,however, the capacitor section 9 is the same as the first capacitorsection 3, which necessitates the female connections at both ends of thesection 8.

A method of use of the apparatus will now be discussed. The primepurpose of the circuit is to develop a characteristic curve over apredetermined range of frequencies depicting the insertion loss of afilter under the worst-case condition for each frequency. As notedhereinbefore, the worst-case condition exists when the reactance of acircuit to which the filter is connected is equal to and opposite invalue to the reactive component of the filter. This is a resonantcondition at the interface, or junction between the filter and thecircuit, and if a high Q is incorporated in the circuits the worst-casecondition is more pronounced. For the purpose of obtaining an adequatetest the Q of the inductors should preferably be or better at and nearthe frequency of the worst-case condition, and other portions of thetest circuit should not insert any appreciable dampening resistance.

A worst-case condition can exist at either end of the filter. Theworst-case with a power source impedance is usually independent of theworst-case with a load impedance, for the reason that at the highfrequencies of electromagnetic induction the impedance of the filter isusually much higher than for either the ower source or load. Theworst-cases at the two ends of the filter are, however, cumulative andtherefore for the purpose of obtaining the total worst-case situationtesting contemplates an impedance mismatch at both filter ends. Ifdesired, of course, the test herein may be conducted with a mismatch onone side alone.

Like inductors are selected for sections 4 and 8, and are assembled withthe other sections 3, 5, 6, 7 and 9. Assembly merely comprises thejoining of each male connector with a female connector of the adjacentsection, and in making such plug-in connections the sealing faces, suchas 29 and 34, are brought up tight with one another to insure that thecavity 1 is an electrically tight enclosure. The connections also havenegligible impedance and are of adequate size to carry the requisite lowfrequency circuit. These objectives are achieved by the hub elementsbeing of large diameter to reduce inductance and to provide currentcarrying capacity. The size of the hubs is substantially larger than thediameter of the inductors ofhigh Q value.

The filter 63 is usually a symmetrical device, presenting a likeimpedance from either of its end terminals 68, and consequentlyidentical inductors in the sections 4 and 6 will produce a likeimpedance mismatch on both sides of the filter 63 and the worst-casecondition will occur on each side at the same frequency.

With the circuit assembled, the transformer 12 and load 15 are regulatedto produce a rated load current of the filter, such as for example 50amperes. The generator 16 is then operated through a range offrequencies with the output current, as measured by probe 19, beingmaintained constant. For the frequency at which the detector 23 measuresa maximum voltage the filter is performing most poorly, and a worst-casecondition exists. At this frequency setting the input current to thetest section 5 is measured by the current probe 19 and recorded. Next,the filter section 6 is removed from the cavity 1. The generator 16 isnow maintained at the same frequency and its output is adjusted to havethe same reading of voltage at the detector 23 as for the worst-casecondition with the filter in the circuit. A reading of the highfrequency input current is again taken with the current probe 19, andthe ratio of the two input current readings, i.e., the current with thefilter in the circuit divided by the current with the filter out of thecircuit, is an indication of insertion loss. To convert such loss intodecibels the following relation is used:

[L 20 log 1 /1 wherein I, is the current reading for the high frequencyinput when the filter is in the circuit and the frequency is adjusted toobtain a maximum voltage across the load impedance of the inductorsection 8, and I, is the current reading for the high frequency inputwhen the filter is removed from the circuit and the frequency andvoltage is maintained the same as when reading 1,.

Next, a second set of inductors is selected and inserted in the circuit.These will give a worst-case condition at a different frequency, and thetest procedure is repeated, to determine a worst-case insertion loss fora second frequency. This procedure is repeated with the use of differentsets of inductor sections, and the values of IL for the differentfrequencies are plotted to obtain a characteristic curve. The accuracyof the procedure is dependent upon the current reading l,, which is thereading of input current with the filter removed, having littlevariation with frequency change in the region of worst-case condition.This usually holds, so that the method herein gives good results. If itdoes not hold for a particular apparatus, then for each set of inductorsa complete insertion loss curve should be plotted in the region of theworst-case condition, and the worst-case points of the individual curvesmust then be connected to obtain the envelope curve representingworstcase conditions.

The invention provides a test apparatus and a method of test for powerline filters that gives a plot of insertion loss for worst-caseconditions in an efficient manner. The interconnectable sectionsfacilitate rapid interchange of parts, and the use of fixed inductorseliminates difficulties of using variable devices which may not haveadequately high Q factorsv The invention further teaches the use of ahigh frequency current source in parallel with the simulated powersource impedance and the use of test leads, in the test sections, thatare in direct contact with the power source-filter circuit.

We claim:

1. in an apparatus for testing electrical filters the Combinationcomprising a test cavity made up of a plurality of plug-in connectedsections, each section having an outer casing with connector elements ateach end and further having inner hubs spaced radially inward of thecasing that are located at the ends of the casing;

one of said sections comprising a chamber for mounting a filter to betested, a second of said sections enclosing a test impedance between theinner hubs of the section that simulates a power source reactance, and athird of the sections mounting a test lead connected with the hubsthereof.

2. In an apparatus for testing electrical filters the combinationcomprising:

the plug-in connection between a pair of sections providing anelectrical junction of negligible impedance between an inner hub of onesection and an inner hub of the other section, and further providing acontinuation of the easing of one section with the casing of the otherto maintain a shielded enclosure;

one of said sections comprising a mounting for a filter to be tested, asecond of said sections mounting a test impedance between the inner hubsof the section, and a third of the sections mounting a test leadconnected with the hubs of the section.

3. An apparatus as in claim 2 in which the test lead is connected in acommon electrical junction with the filter and the test impedance.

4. An apparatus as in claim 2 having a feed-in capacitor section forconnection to the section mounting the test impedance for introducing alow frequency load current to the apparatus.

5. An apparatus as in claim 2 wherein said test impedance has a Q of atleast 15 for the frequency of worst-case condition.

6. A section for a test apparatus that is connectable with additionalsections, such section comprising:

an outer tubular casing;

a female connector ring fitted in one end of said casing having an axialfacing sealing face and a socket portion adjacent such sealing face;

a male connector ring fitted in the other end of said casing having anaxial facing sealing face and a ring portion adjacent such sealing facefor receipt by a socket portion of a female connector ring;

a radially extending supporting insulator mounted in each end of saidcasing;

a female inner hub supported by one of said insulators that is spacedradially inward from said casing, which hub has a socket portion and isof such radial extent as to present negligible impedance;

a male inner hub supported by the other of said insulators that isspaced radially inward from said casing, which hub has a rim portion forreceipt by a socket portion of a female inner hub and is of such radialextent as to present negligible impedance; and

circuit means between said inner hubs.

7. A device as in claim 6 wherein the circuit means is an inductorhaving a Q of at least 15 at the frequency of worst-case condition.

8. A device as in claim 6 wherein the circuit means is a filter to betested.

9. A device as in claim 6 wherein the circuit means is a lead in directcontact with the hubs.

10. A circuit for testing insertion loss of electrical filterscomprising:

a first impedance element representative of a power source impedance;

a high frequency current source in an electrical parallel connectionwith said impedance element that comprises a direct metal to metalcontact; a

a second impedance element representative of a load impedance;

connections for inserting and removing a filter from between said secondimpedance element and the parallel connection of said first impedanceelement and said current source, said connections also joining thefilter in parallel with both said first and second impedances; and

test connections for reading electrical values in the circuit that arein electrical parallel relation with said second impedance.

11. A circuit as in claim 10, wherein said test connections are betweenthe second impedance element and connections for a filter, to therebyread voltage of the second impedance.

12. A circuit as in claim 10 having a pair of lead-in capacitors, oneconnected to the first impedance and the other connected to the secondimpedance, said capacitors having feedthrough lines for introducing alow frequency load current to a filter under test, and the capacitiveelements providing a bypass for the high frequency.

13. A circuit as in claim 12 wherein the impedance elements and filterare within a closed cavity and said lead-in capacitors are at the endsof the cavity.

14. in a test circuit for electrical filters the combination of:

an enclosed cavity comprising a filter section having an outer shieldand a mounting therein for a filter to be tested, a first inductorsection having an outer shield and an inductor therein that isconnectable with said filter and that simulates a power sourceimpedance, a first test section having an outer casing and a feed-inlead connectable by direct metallic connection with the commonconnection of said filter and inductor to place the feed-in lead,inductor and a filter to be tested in electrical parallel relation withone another, a second inductor section having an outer casing and asecond inductor therein that is connectable to said filter and thatsimulates a load impedance, and a second test section having an outershield and a feed-in lead connectable by direct metallic connection withsaid second inductor to place the feed-in lead, second inductor and afilter to be tested in electrical parallel relation with one another;

a high frequency generator joined to tee feed-in lead of said first testsection to supply an electromagnetic interference simulating current;and

a detector connected to the feed-in lead of said second test section toread load voltage; and

the outer shields of the aforesaid sections joining one another to formsaid cavity.

15. A circuit as in claim 14 wherein said inductor has a Q of at least15 at the frequency of worst-case condition.

16. A test circuit as in claim 14 having a feed-in capacitor connectedto each inductor section, each feed-in capacitor having a shieldedexterior that forms a part of the cavity and a feed-through line forintroducing low frequency load current to a filter under test, theplates of each capacitor forming a bypass for high frequency testsignals.

)7. ln an apparatus for testing electrical filters, the combinationcomprising:

a feed-in capacitor section having a shielded exterior and afeed-through line with a capacitive element between such line andexterior, said capacitor section having a connector ring encircling aterminus of said feed-through line that presents a sealing face forabutment with a subsequent section of the apparatus, and further havinga connection hub mounted within said connector ring that is secured tosaid feed-through line;

an inductor section having an outer casing, a ring member at each end ofsaid casing that presents a sealing face for abutment with a similarface of an adjacent section, a radially inward extending supportinginsulator mounted within each end of said casing, an inner hub supportedby each insulator which is substantially concentric with said casing,and an inductor connected between said inner hubs of a cross sectiondimension only a-minor fraction of the cross section of said hubs;

' said inductor section and said capacitor section having a plug-inconnection with one another, wherein the ring and inner hub at one endof said inductor section form an electrical junction with the connectorring and connection hub of said capacitor section, parts of one ringbeing a male member received by a female member in the other ring, andparts of one hub being a male member received by a female member in theother hub, and the sealing faces of the joining rings abutting againstone another, the resulting electrical junction providing a negligibleimpedance connection between hubs and a continuous cavity betweensections;

a test section having an outer casing, a ring member at each end of saidcasing that presents a sealing face for abutment with a similar face ofan adjacent section, a pair of radially inward extending supportinginsulators mounted within said casing, an inner hub supported by eachinsulator which is substantially concentric with said casing, said hubsbeing joined as a continuation of one another. and a test lead extendingfrom said hubs radially outward through said outer casing;

said inductor section and said probe section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of said testsection form an electrical junction with a ring and inner hub at an endof said inductor section, parts of one ring being a male member receivedby a female member in the other ring, and parts of one hub being a malemember received by a female member in the other hub, and the sealingfaces of the joining rings abutting against one another, the resultingelectrical junction providing a negligible impedance connection betweenhubs and a continuous cavity between sections;

a filter section having an outer casing, a ring member at each end ofsaid casing that presents a sealing face for abutment with a similarface of an adjacent section, a mounting web in-the casing in which afilter to be tested is mounted. and a inner hub supportable at each endof the filter which is substantially concentric with said casing;

said filter section and said test section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of saidfilter section form an electrical junction with the connector ring andconnection hub of said test section, parts of one ring being a malemember received by a female member in the other ring, and parts of onehub being a male member received by a female member in the other hub,and the sealing faces of the joining rings abutting against one another,the resulting electrical junction providing a negligible impedanceconnection between hubs and a continuous cavity between sections; and

second test, inductor and feed-in capacitor sections similar to theaforesaid test, inductor and feed-in capacitor sections connected to oneanother on the opposite end of the filter section.

18. in an apparatus for testing electrical filters, the combinationcomprising:

a feed-in capacitor section having a shielded exterior and afeed-through line with a capacitive element between such line andexterior, said capacitor section having a connector ring encircling saidfeed-through line and a connection hub secured to said feed-throughline;

an inductor section having an outer casing, a ring member at each end ofsaid casing, a supporting insulator mounted within each end of saidcasing, an inner hub supported by each insulator, and an inductorconnected between said inner hubs;

said inductor section and said capacitor section having a plug-inconnection with one another, wherein the ring and inner hub at one endof said inductor section form an electrical junction with the connectorring and connection hub of said capacitor section, parts of one ringbeing a male member received by a female member in the other ring, andparts of one hub being a male member received by a female member in theother hub;

a test section having an outer casing, a ring member at each end of saidcasing, a pair of supporting insulators mounted within said casing, aninner hub supported by each insulator, said hubs being joined as acontinuation of one another, and a test lead extending from said hubsradially outward through said outer casing;

said inductor section and said test section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of said testsection form an electrical junction with a ring and inner hub at an endof said inductor section, parts of one ring being a male member receivedby a female member in the other ring, and parts of one hub being a malemember received by a female member in the other hub;

a filter section having an outer casing, a ring member at each end ofsaid casing, a mounting web in the casing in which a filter to be testedis mounted, and an inner hub supportable at each end of the filter;

said filter section and said test section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of saidfilter section form an electrical junction with the connector ring andconnection hub of said test section, parts of one ring being a malemember received by a female member in the other ring, and parts of onehub being a male member received by a female member in the other hub,and

second test, inductor and feed'in capacitor sections similar to theaforesaid test, inductor and feed-in capacitor sections connected to oneanother on the opposite end of the filter section.

19. in an apparatus for testing electrical filters, the combinationcomprising:

an inductor section having an outer casing, a ring member at each end ofsaid casing, an inner hub supported in each end of and spaced from saidcasing, and an inductor connected between said inner hubs of a crosssection only a minor fraction of the cross section of said hubs;

a test section having an outer casing, a ring member at each end of saidcasing, an inner hub supported in each end of and spaced from saidcasing, said hubs being joined as a continuation of one another, and atest lead extending from said hubs radially outward through said outercasing;

said inductor section and said test section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of said testsection form an electrical junction with a ring and inner hub at an endof said inductor section, parts of one ring being a male member receivedby a female member in the other ring, and parts of one hub being a malemember received by a female member in the other hub;

a filter section having an outer casing, a ring member at each end ofsaid casing, a mounting web in the casing in which a filter to be testedis mounted, and an inner hub supportable at each end of the filter; and

said filter section and said test section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of saidfilter section form an electrical junction with the connector ring andconnection hub of said test section, parts of one ring being a malemember received by a female member in the other ring, and parts of onehub being a male member received by a female member in the other hub.

20. A method of testing a filter comprising:

attaching a high frequency power source and an inductor in arallel andattaching the parallel connection to a filter to be tested;

attaching a second inductor to the opposite side of the filter;

maintaining power source current at a fixed level and varying thefrequency of the output of the power source to select that frequency atwhich a maximum voltage is detected for the second inductor;

removing the filter from the circuit;

maintaining the selected frequency and the maximum voltage across thesecond inductor;

reading the power source current;

and taking a comparison of the power source current reading which wasmaintained when the filter was in the circuit with the current readingwith the filter out of the circuit to thereby obtain an indication ofthe insertion loss of the filter for the selected frequency.

21. A method of testing a filter comprising:

joining the filter between two inductors simulating a power sourcereactance and a load reactance;

feeding a high frequency current to the filter and power sourcereactance;

selecting a current level for the high frequency, l and varying thefrequency to obtain a maximum voltage for the load reactance;

removing the filter from the circuit;

for the same maximum voltage and the same frequency at which suchmaximum voltage was obtained measuring the high frequency current, 1,,

comparing I with I, as an indication of filter insertion loss mg UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 651 400Dated March 21, 1972 lnventofls) William J. McMartin and Hans E.Weidmann It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 3, "or" should read -of- Column 4, line 6, "tat" shouldread ---that--- Column 4, line 34, after "32" insert --is- Column 4,line 73, after "4, insert ---5.--- Column 8, line 25, "tee" should read---the--- Column 9, line 22, "a" should r ad Column line 1, deletesecond appearance of Signed and sealed this 18th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISCHALK Attesting Officer Commissionerof Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3 51 400 Dated March 21 1972 Inventor) William J McMartin et a1 Itis certified that error appears in the aboveidentified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 8 line 32, after "inductor" insert of said first inductor sectionSigned and sealed this 12th day of September 1972 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-IOSO (IO-69) USCOMM DC o376 p9 0 u s covnmuzurWHNYING ornct I90 o sssuu

1. In an apparatus for testing electrical filters the combinationcomprising a test cavity made up of a plurality of plug-in connectedsections, each section having an outer casing with connector elements ateach end and further having inner hubs spaced radially inward of thecasing that are located at the ends of the casing; one of said sectionscomprising a chamber for mounting a filter to be tested, a second ofsaid sections enclosing a test impedance between the inner hubs of thesection that simulates a power source reactance, and a third of thesections mounting a test lead connected with the hubs thereof.
 2. In anapparatus for testing electrical filters the combination comprising: atest cavity made up of a plurality of plug-in connected sections, eachsection having an outer casing with connector elements at each end andfurther having inner connector hubs spaced radially inward of the casingthat are located at the ends of the casing; the plug-in connectionbetween a pair of sections providing an electrical junction ofnegligible impedance between an inner hub of one section and an innerhub of the other section, and further providing a continuation of thecasing of one section with the casing of the other to maintain ashielded enclosure; one of said sections comprising a mounting for afilter to be tested, a second of said sections mounting a test impedancebetween the inner hubs of the section, and a third of the sectionsmounting a test lead connected with the hubs of the section.
 3. Anapparatus as in claim 2 in which the test lead is connected in a commonelectrical junction with the filter and the test impedance.
 4. Anapparatus as in claim 2 having a feed-in capacitor section forconnection to the section mounting the test impedance for introducing alow frequency load current to the apparatus.
 5. An apparatus as in claim2 wherein said test impedance has a Q of at least 15 for the frequencyof worst-case condition.
 6. A section for a test apparatus that isconnectable with additional sections, such section comprising: an outertubular casing; a female connector ring fitted in one end of said casinghaving an axial facing sealing face and a socket portion adjacent suchsealing face; a male connector ring fitted in the other end of saidcasing having an axial facing sealing face and a ring portion adjacentsuch sealing face for receipt by a socket portion of a female connectorring; a radially extending supporting insulator mounted in each end ofsaid casing; a female inner hub supported by one of said insulators thatis spaced radially inward from said casing, which hub has a socketportion and is of such radial extent as to present negligible impedance;a male inner hub supported by the other of said insulators that isspaced radially inward from said casing, which hub has a rim portion forreceipt by a socket portion of a female inner hub and is of such radialextent as to present negligible impedance; and circuit means betweensaid inner hubs.
 7. A device as in claim 6 wherein the circuit means isan inductor having a Q of at least 15 at the frequency of worst-casecondition.
 8. A device as in claim 6 wherein the circuit means is afilter to be tested.
 9. A device as in claim 6 wherein the circuit meansis a lead in direct contact with the hubs.
 10. A circuit for testinginsertion loss of electrical filters comprising: a first impedanceelement representative of a power source impedance; a high frequencycurrent source in an electrical parallel connection with said impedanceelement that comprises a direct metal to metal contact; a secondimpedance element representative of a load impedance; connections forinserting and removing a filter from between said second impedanceelement and the parallel connection of said first impedance element andsaid current source, said connections also joining the filter inparallel with both said first and second impedances; and testconnections for reading electrical values in the circuit that are inelectrical parallel relation with said second impedance.
 11. A circuitas in claim 10, wherein said test connections are between the secondimpedance element and connections for a filter, to thereby read voltageof the second impedance.
 12. A circuit as in claim 10 having a pair oflead-in capacitors, one connected to the first impedance and the otherconnected to the second impedance, said capacitors having feed-throughlines for introducing a low frequency load current to a filter undertest, and the capacitive elements providing a bypass for the highfrequency.
 13. A circuit as in claim 12 wherein the impedance elementsand filter are within a closed cavity and said lead-in capacitors are atthe ends of the cavity.
 14. In a test circuit for electrical filters thecombination of: an enclosed cavity comprising a filter section having anouter shield and a mounting therein for a filter to be tested, a firstinductor section having an outer shield and an Inductor therein that isconnectable with said filter and that simulates a power sourceimpedance, a first test section having an outer casing and a feed-inlead connectable by direct metallic connection with the commonconnection of said filter and inductor to place the feed-in lead,inductor and a filter to be tested in electrical parallel relation withone another, a second inductor section having an outer casing and asecond inductor therein that is connectable to said filter and thatsimulates a load impedance, and a second test section having an outershield and a feed-in lead connectable by direct metallic connection withsaid second inductor to place the feed-in lead, second inductor and afilter to be tested in electrical parallel relation with one another; ahigh frequency generator joined to tee feed-in lead of said first testsection to supply an electromagnetic interference simulating current;and a detector connected to the feed-in lead of said second test sectionto read load voltage; and the outer shields of the aforesaid sectionsjoining one another to form said cavity.
 15. A circuit as in claim 14wherein said inductor has a Q of at least 15 at the frequency ofworst-case condition.
 16. A test circuit as in claim 14 having a feed-incapacitor connected to each inductor section, each feed-in capacitorhaving a shielded exterior that forms a part of the cavity and afeed-through line for introducing low frequency load current to a filterunder test, the plates of each capacitor forming a bypass for highfrequency test signals.
 17. In an apparatus for testing electricalfilters, the combination comprising: a feed-in capacitor section havinga shielded exterior and a feed-through line with a capacitive elementbetween such line and exterior, said capacitor section having aconnector ring encircling a terminus of said feed-through line thatpresents a sealing face for abutment with a subsequent section of theapparatus, and further having a connection hub mounted within saidconnector ring that is secured to said feed-through line; an inductorsection having an outer casing, a ring member at each end of said casingthat presents a sealing face for abutment with a similar face of anadjacent section, a radially inward extending supporting insulatormounted within each end of said casing, an inner hub supported by eachinsulator which is substantially concentric with said casing, and aninductor connected between said inner hubs of a cross section dimensiononly a minor fraction of the cross section of said hubs; said inductorsection and said capacitor section having a plug-in connection with oneanother, wherein the ring and inner hub at one end of said inductorsection form an electrical junction with the connector ring andconnection hub of said capacitor section, parts of one ring being a malemember received by a female member in the other ring, and parts of onehub being a male member received by a female member in the other hub,and the sealing faces of the joining rings abutting against one another,the resulting electrical junction providing a negligible impedanceconnection between hubs and a continuous cavity between sections; a testsection having an outer casing, a ring member at each end of said casingthat presents a sealing face for abutment with a similar face of anadjacent section, a pair of radially inward extending supportinginsulators mounted within said casing, an inner hub supported by eachinsulator which is substantially concentric with said casing, said hubsbeing joined as a continuation of one another, and a test lead extendingfrom said hubs radially outward through said outer casing; said inductorsection and said probe section having a plug-in connection with oneanother, wherein the ring and inner hub at one end of said test sectionform an electrical junction with a ring and inner hub at an end of saidinductor section, parts of one ring being a male member received by afEmale member in the other ring, and parts of one hub being a malemember received by a female member in the other hub, and the sealingfaces of the joining rings abutting against one another, the resultingelectrical junction providing a negligible impedance connection betweenhubs and a continuous cavity between sections; a filter section havingan outer casing, a ring member at each end of said casing that presentsa sealing face for abutment with a similar face of an adjacent section,a mounting web in the casing in which a filter to be tested is mounted,and a inner hub supportable at each end of the filter which issubstantially concentric with said casing; said filter section and saidtest section having a plug-in connection with one another, wherein thering and inner hub at one end of said filter section form an electricaljunction with the connector ring and connection hub of said testsection, parts of one ring being a male member received by a femalemember in the other ring, and parts of one hub being a male memberreceived by a female member in the other hub, and the sealing faces ofthe joining rings abutting against one another, the resulting electricaljunction providing a negligible impedance connection between hubs and acontinuous cavity between sections; and second test, inductor andfeed-in capacitor sections similar to the aforesaid test, inductor andfeed-in capacitor sections connected to one another on the opposite endof the filter section.
 18. In an apparatus for testing electricalfilters, the combination comprising: a feed-in capacitor section havinga shielded exterior and a feed-through line with a capacitive elementbetween such line and exterior, said capacitor section having aconnector ring encircling said feed-through line and a connection hubsecured to said feed-through line; an inductor section having an outercasing, a ring member at each end of said casing, a supporting insulatormounted within each end of said casing, an inner hub supported by eachinsulator, and an inductor connected between said inner hubs; saidinductor section and said capacitor section having a plug-in connectionwith one another, wherein the ring and inner hub at one end of saidinductor section form an electrical junction with the connector ring andconnection hub of said capacitor section, parts of one ring being a malemember received by a female member in the other ring, and parts of onehub being a male member received by a female member in the other hub; atest section having an outer casing, a ring member at each end of saidcasing, a pair of supporting insulators mounted within said casing, aninner hub supported by each insulator, said hubs being joined as acontinuation of one another, and a test lead extending from said hubsradially outward through said outer casing; said inductor section andsaid test section having a plug-in connection with one another, whereinthe ring and inner hub at one end of said test section form anelectrical junction with a ring and inner hub at an end of said inductorsection, parts of one ring being a male member received by a femalemember in the other ring, and parts of one hub being a male memberreceived by a female member in the other hub; a filter section having anouter casing, a ring member at each end of said casing, a mounting webin the casing in which a filter to be tested is mounted, and an innerhub supportable at each end of the filter; said filter section and saidtest section having a plug-in connection with one another, wherein thering and inner hub at one end of said filter section form an electricaljunction with the connector ring and connection hub of said testsection, parts of one ring being a male member received by a femalemember in the other ring, and parts of one hub being a male memberreceived by a female member in the other hub, and second test, inductorand feed-in capacitor sections similar to the aforesaid test, inductorand fEed-in capacitor sections connected to one another on the oppositeend of the filter section.
 19. In an apparatus for testing electricalfilters, the combination comprising: an inductor section having an outercasing, a ring member at each end of said casing, an inner hub supportedin each end of and spaced from said casing, and an inductor connectedbetween said inner hubs of a cross section only a minor fraction of thecross section of said hubs; a test section having an outer casing, aring member at each end of said casing, an inner hub supported in eachend of and spaced from said casing, said hubs being joined as acontinuation of one another, and a test lead extending from said hubsradially outward through said outer casing; said inductor section andsaid test section having a plug-in connection with one another, whereinthe ring and inner hub at one end of said test section form anelectrical junction with a ring and inner hub at an end of said inductorsection, parts of one ring being a male member received by a femalemember in the other ring, and parts of one hub being a male memberreceived by a female member in the other hub; a filter section having anouter casing, a ring member at each end of said casing, a mounting webin the casing in which a filter to be tested is mounted, and an innerhub supportable at each end of the filter; and said filter section andsaid test section having a plug-in connection with one another, whereinthe ring and inner hub at one end of said filter section form anelectrical junction with the connector ring and connection hub of saidtest section, parts of one ring being a male member received by a femalemember in the other ring, and parts of one hub being a male memberreceived by a female member in the other hub.
 20. A method of testing afilter comprising: attaching a high frequency power source and aninductor in parallel and attaching the parallel connection to a filterto be tested; attaching a second inductor to the opposite side of thefilter; maintaining power source current at a fixed level and varyingthe frequency of the output of the power source to select that frequencyat which a maximum voltage is detected for the second inductor; removingthe filter from the circuit; maintaining the selected frequency and themaximum voltage across the second inductor; reading the power sourcecurrent; and taking a comparison of the power source current readingwhich was maintained when the filter was in the circuit with the currentreading with the filter out of the circuit to thereby obtain anindication of the insertion loss of the filter for the selectedfrequency.
 21. A method of testing a filter comprising: joining thefilter between two inductors simulating a power source reactance and aload reactance; feeding a high frequency current to the filter and powersource reactance; selecting a current level for the high frequency, I11, and varying the frequency to obtain a maximum voltage for the loadreactance; removing the filter from the circuit; for the same maximumvoltage and the same frequency at which such maximum voltage wasobtained measuring the high frequency current, I2, comparing I1 with I2as an indication of filter insertion loss for the said frequency.
 22. Amethod as in claim 21, in which the comparison of I1 and I2 is made bythe following relationship: IL 20 log I1/I2 and repeating the procedureseveral times with different reactance values to obtain a characteristiccurve over a range of frequencies.